In the manufacture of optical fiber, a glass preform rod is suspended vertically and moved into a furnace at a controlled rate. The preform softens in the furnace and a glass fiber (also referred to as an optical fiber) is drawn freely from the molten end of the preform rod by a capstan located at the base of a draw tower. Because the surface of the glass fiber is susceptible to damage caused by abrasion, it is necessary to coat the fiber after it is drawn but before it comes into contact with any surface. Inasmuch as the application of a coating material must not damage the glass surface, the coating material is applied in a liquid state. Once applied, the coating material must solidify before the glass fiber reaches the capstan. This is typically accomplished within a brief time interval by photocuring--a process in which the liquid coating material is converted to a solid by exposure to electromagnetic radiation.
Because the fibers are thin and flexible, they are readily bent when subjected to mechanical stresses such as those encountered during handling or exposure to varying temperature environments. Such bends in the fiber frequently result in loss that is much greater than the intrinsic loses of the fiber itself, and it has been found desirable to protect the glass fiber against such bending. Accordingly, the coating material is required to cushion the glass fiber against bends and two layers of coating materials are typically applied to the drawn optical fiber. An inner (primary) coating, having a relatively low modulus, is applied directly to the glass fiber; and an outer (secondary) coating, having a relatively high modulus, surrounds the primary coating. Together, these coatings desirably protect the inherently high tensile strength of the glass fiber so long as the primary coating material remains bonded to the glass. However, in what appears to be a contradictory requirement, it is also desirable to be able to easily strip the coating(s) from the glass fiber--particularly when a number of fibers are bonded together in an array such as shown in U.S. Pat. No. 4,900,126 which leads to yet another performance attribute that coating materials need to possess. Indeed, if the coating materials cannot be cleanly and easily stripped, then splicing and connectorizing operations will be seriously hampered.
U.S. Pat. No. 4,472,021 discloses a strippable coating for an optical fiber which comprises from about 2% to about 20% of an organic polysiloxane additive having a plurality of hydroxy-terminated groups which are joined to some of the silicon atoms in the polysiloxane chain by a carbon-to-silicon bond. This particular additive, however, exhibits low adhesion to the glass fiber which adversely affects the handling properties of of the coated optical fiber during the manufacturing process. Low adhesion is a problem which, in the worst case, leads to "delamination" and the ensuing incursion of water, particularly upon exposure to high humidity, which attacks the glass surface and reduces tensile strength.
What is needed, and seemingly what is not disclosed in the prior art, is an additive for use in a coating for optical fibers which improves strippability while still maintaining adhesion to the glass fiber.